1. Field of the Invention
The present invention relates to a scintillator panel converting radiation rays to visible light and a radiation detector using the scintillator panel.
2. Description of the Related Art
Recent digitalized radiation detectors, such as those for medical tests or industrial nondestructive inspections, are mainly systems employing a method of converting incident X rays to visible light in a scintillator layer such as computed radiology (hereinafter, referred to as CR) and flat panel detectors (hereinafter, referred to as FPD).
Europium-added cesium bromide (CsBr:Eu) used as the phosphor layer in some CR systems and thallium-added cesium iodide (CsI:Tl) used in most FPDS are materials commonly used because they often have columnar crystals when produced by the vacuum deposition method.
For example, scintillator panels using CsI:Tl are usually produced by coating a reflective film on a radiation ray-permeable supporting substrate such as glass and forming a CsI film thereon. A protection film is occasionally formed additionally between the reflective and CsI films for protection of the reflective film.
An X ray entering the scintillator panel having such a structure through a subject from an X-ray source is converted into visible light by the scintillator. For example in the case of an X-ray photon, the photon is converted into visible light at the emission point in the phosphor layer. The light emitted at the emission point disperses in all directions, independently of the vector of the incident photon. Because the phosphor layer has a pillar structure, some of the emitted photons disperse out of the surface of the scintillator panel through the pillars, because of the difference in refractive index between pillars and the CsI layer (refractive index of CsI: 1.8). The light dispersed farther than the adjacent pillar is considered unlikely to travel through the optical interface between the many pillars in the surface direction of the phosphor layer, and thus is entrapped in one of the pillars at the interface and disperses out of the surface of the scintillator panel through the pillar. For the reasons above, the phosphor layer having a pillar structure has a function to transmit the emitted light to the next device (e.g., photodiode in the case of FPD) without light scattering, giving a scintillator layer having high-definition images.
The reflective film has a function to reflect the emitted light traveling in the supporting substrate direction to the CsI surface, and thus to improve the sensitivity of the scintillator panel.
For example, an FPD having a phosphor layer has a shape in which the scintillator panel is bonded to an image sensor having multiple photoreceptor elements arranged in a one- or two-dimensional array. The definition and sensitivity characteristics of FPDs in such a structure are influenced by the properties of the scintillator panel. In other words, the characteristics of the FPD are dependent on the pillar structure of CsI and the function of the reflective film (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-58099, pp. 4 to 5 and FIG. 3).